DESCRIPTION

LibGG implements a task scheduler in both threaded and non-threaded
environments. Tasks can be registered with the scheduler to run short,
asynchronous routines called "handlers" which may interrupt or run in
parallel with the normal flow-of-control. It is recommended to use
LibGG tasks in lieue of threads when writing for maximum portability,
if they can meet the demands of the application, since not all
environments support threads.
The LibGG task scheduler uses a unit of time called a "tick", which may
vary between architectures. The tick is guaranteed to be no more than
one second, however, most environments will support at least 60 ticks
per second. By default LibGG will select 60 ticks per second if it is
supported, see below for instructions on modifying this behavior. The
function ggTimeBase is used to find out the size of a tick.
GG_SCHED_TICKS2USECS and GG_SCHED_USECS2TICKS are convenient macros
that simplifies conversion between ticks and microseconds and vice
versa.
The maximum rate at which a periodic task may run is once per tick.
The maximum period (minimum rate) of a LibGG task is the value of the
macro GG_SCHED_TICK_WRAP minus one, and is also measured in ticks.
ggAddTask will examine the values in the offered task control structure
task. Before calling ggAddTask the task control structure must be
initialized by filling it with zeros, including the internal-use-only
area. The task control structure should be further initialized by
providing at least a pointer to a callback handler function in the
member cb, and initializing the pticks member to contain the number of
ticks between each call to the handler function. The ncalls member may
be left at zero, in which case the task remains scheduled to run once
every pticks until explicitly deleted, or it may be set to a positive
integer to indicate that the task should be automatically deleted after
the handler has been called ncalls times. The int return type on the
callback hook is only there for possible future expansion. For now
callbacks should always return 0. Other values are undefined.
The task control structure must only be used for one task, however a
task handler may be called by multiple tasks. The member hook is
provided for the application’s use in the task control structure as a
means to easily transport task-local data to the handler. If a tick
arrives during a call to ggAddTask, the handler may be invoked before
ggAddTask returns; A memory barrier is included in ggAddTask which
ensures that all values in the task control structure are up to date on
multiprocessor systems even in this case. The task control structure
should not be altered, except by a task handler as noted below, while
the task is scheduled.
ggDelTask will remove a task from the scheduler. The task may be
called after ggDelTask is called, but is guaranteed not to be called
after ggDelTask has returned, until such a point as it is added again
with ggAddTask.
A task can be put to sleep for a certain amount of time in microseconds
by altering the period of the task to the correct number of ticks, and
then that task itself can reset it’s period back based on a value in
it’s private hook when it next runs.
A task can wait for an other task to finish either by writing code to
poll the other task’s flags, or by writing a callback into the latter
task when it is done to reschedule a list of waiting tasks. How a task
terminates is entirely up to the author.
Each scheduled task is guaranteed never to be reentered by the
scheduler. That is, only one call to a task handler for a given task
control structure will be run at a time, though a single handler
function that handles more than one task control structure may be
entered simultaneously once per structure.
When a task executes, the handler is invoked and the parameter task
given to the handler contains the same pointer value as was given to
ggAddTask. The ncalls member will be updated to contain the number of
calls, including the current call, which remain before the task is
automatically deleted (or zero if the task will never be automatically
deleted.) Thus it is safe to call ggAddTask again to reuse the task
control structure once the handler has returned with ncalls equal to 1.
The lasttick member will contain the number of the LibGG scheduler tick
being executed, which should increase monotonically unless a problem
occurs as noted below, wrapping around modulus the value
GG_SCHED_TICK_WRAP.
ggAddTask and ggDelTask may not be called from within a task handler,
however, the task handler is free to alter the pticks and ncalls
members in the task control structure task in order to change its
period, or increase or decrease the number of calls before auto-
deletion. For example, to cancel itself, a task need only set ncalls
to 1 before returning. The task handler may also change it’s callback
function or data hook members. A write memory barrier is included in
the scheduler to prevent old values from being seen by other processors
on SMP systems.
LibGG ticks are measured in real (wall clock) time and LibGG makes
every effort to ensure that drift due to runtime factors is kept at a
minimum. When a process is suspended, however, LibGG ticks stop and
resume where they left off. Likewise, when system utilization is very
high or tasks are misused the LibGG scheduler may fail to count ticks.
However the ggCurTime(3) function will still be accurate in these cases
and can be used to detect such situations.
All scheduled LibGG tasks may in the worst case have to be run
serialized, and may be postponed slightly while a call to ggAddTask or
ggDelTask is in progress, so there may be some delay between the start
of a LibGG tick and the actual execution of the task. This can be
minimized by limiting the duties of task handlers to very short, quick
operations.
When utilization is high or tasks misbehave, the scheduler may elect
simply not to call a task handler even though it is scheduled to be
called on a given tick. This may happen either to all tasks or to
select individual tasks. The "lasttick" member of the task control
structure can be safely read from within a task handler in order to
detect such a circumstance (it will always contain the current tick,
but can be compared to a previously stored value.)
Since LibGG tasks may be called in a signal handler or other non-
interruptible context, they should not call ggLock(3) on any locks that
may already be locked. In addition, there may be limits imposed on the
functions which are safe to use inside task handlers (that is, only
reentrant functions may be safe.) More detailed information on using
locks inside LibGG task handlers is contained in the manpage for
ggLock(3).
Scheduled tasks will be canceled, in a somewhat precarious fashion, by
a normal call to ggExit(3). As such, it is considered best practice to
use ggDelTask to cancel tasks when gracefully deinitializing LibGG or a
library that uses LibGG.

RETURNVALUE

ggAddTask returns GGI_OK on success or:
· GGI_EARGREQ if called with NULL argument;
· GGI_EARGINVAL if the task is incorrectly set;
· GGI_EBUSY if the task is already added;
· GGI_ENOMEM if the task lock could not be created.
ggDelTask returns GGI_OK on success or:
· GGI_EARGREQ if called with NULL argument;
· GGI_EARGINVAL if the task is not currently scheduled.
ggTimeBase returns an integer between 1 and 1000000, inclusive, which
represents the number on microseconds between each tick of the LibGG
scheduler.

ENVIRONMENTVARIABLES

If the "-schedhz=speed" option is present in the GG_OPTS environment
variable when ggInit is first called, the scheduler time base will be
set such that the scheduler executes speed ticks per second. If this
is not possible, ggInit(3) will fail. The default speed is 60HZ, or
the maximum that the environment can support, whichever is less.
If the "-signum=n" option is present in the GG_OPTS environment
variable when ggInit is first called, and LibGG is not compiled with
threads support, the UNIX signal used by the scheduler may be selected.
If n is not a valid signal for this purpose, the results are undefined,
but should not be unsafe for SUID processes. The default signal used
is usually SIGPROF, but may be chosen differently based on the needs of
the package maintainer for any particular LibGG distribution.
Applications using LibGG are forbidden from using this signal for other
purposes, whether or not tasks are used.
If the "-schedthreads=numthreads" option is present in the GG_OPTS
environment variable when ggInit is first called, and LibGG is compiled
with threading support, the scheduler will create numthreads additional
threads to call task handlers. The default is one additional thread.
If numthreads is not valid or causes resource allocation problems, the
results are undefined, but should not be unsafe for SUID (or other
elevated privilege) processes.